In UMTS (Universal Mobile Telecommunications System) scheme that is one of the third generation mobile communication schemes (3G), HSDPA (High Speed Downlink Packet Access) is standardized as a scheme for realizing high speed packet transmission in a link (to be referred to as a downlink hereinafter) from a base station to a mobile station.
In the HSDPA, for increasing throughput that can be realized, a packet scheduling technique is adopted in which users connecting to the base station share radio resources (to be referred to as a shared packet channel hereinafter), and the base station assigns the radio resources to a user having good propagation state on a priority basis.
In HSDPA, single carrier signal transmission is performed in a 5 MHz channel bandwidth. Thus, the entire channel bandwidth of 5 MHz is used for data that is signal-transmitted by the shared packet channel, and assignment of transmission slots to each user is performed based on time multiplexing basically. In addition, also for a control channel for reporting which user is assigned the transmission slot and the like for realizing this shared packet channel transmission, signal transmission is performing using the entire channel bandwidth of 5 MHz.
On the other hand, consideration of standardization on long term evolution (LTE) of the UMTS is started, and study of the fourth generation mobile communication scheme (4G) that is a mobile communication scheme of a next generation of 3G is being progressed. In the LTE of 3G and the 4G schemes, it is desired to flexibly support from a multi-cell environment such as cellular systems to an isolated-cell environment such as hot spot areas and indoor areas, and further desired to increase frequency use efficiency for both of the cell environments.
As a radio access scheme adopted for a downlink in the LTE of 3G and the 4G schemes, it is considered that OFDM (Orthogonal Frequency Division Multiplexing) for transmitting signals using many sub-carriers is a strong candidate (refer to non-patent document 1, for example).
In the OFDM, serial to parallel conversion is performed on a data series to be transmitted, so that parallel signals of the data series are transmitted using a plurality of sub-carriers. Accordingly, since a symbol rate becomes low, effects of delayed waves (multi-path) caused due to difference of propagation routes from a transmitter to a receiver is suppressed, so that it becomes possible to transmit a signal of high information rate with high quality.
In addition, for supporting the multi-cell environment by the radio access scheme using the OFDM, it is desirable to apply one-cell frequency reuse for realizing large capacity. For realizing the one-cell frequency reuse, application of spreading is effective to suppress effects of interference signals using the same frequency from surrounding adjacent cells.
Thus, in OFDM (OFDM with Spreading) to which spreading is applied, as shown in FIGS. 1A and 1B, after the input data is channel-coded and data-modulated, spreading is applied, and serial to parallel conversion and inverse Fourier conversion are performed, so that signals of multi-carriers are generated and are transmitted after a guard interval is inserted. More particularly, as a case where spreading is applied, when 8 is applied as a spreading factor for example, each symbol is spread into eight sub-carries and is transmitted. In addition, when effects of interference from surrounding cells are small, 1 is applied as a spreading factor since it is unnecessary to apply spreading, so that data D1, D2, . . . that are different in each sub-carrier are transmitted.
Also in OFDM or OFDM to which spreading is applied as mentioned above, the packet scheduling technique can be applied using a shared channel like one used in HSDPA, and throughput can be increased accordingly. In this case, since multi-carrier transmission is performed in OFDM for shared channel transmission, not only a method for performing transmission slot assignment to each user by time multiplexing like HSDPA is available but also it is available to assign radio resources to each user in units of a sub-carrier or in units of a frequency block in which a plurality of sub-carriers are bound (refer to non-patent document 2, for example).
Therefore, when performing signal transmission using multi-carrier transmission within a channel bandwidth, a method for assigning radio resources different from one for HSDPA becomes possible.
However, in order to realize actual mobile communication in OFDM or OFDM using spreading, in addition to transmitting the shared channel, it is necessary to transmit a control channel necessary for transmitting control information of the shared channel or transmit a common control channel for transmitting system information and paging information that are transmitted to all users connected to a base station.
[Non-patent document 1] J. A. C. Bingham, “Multicarrier modulation for data transmission: an idea whose time has come,” IEEE Commun. Mag., pp. 5-14, May 1990.
[Non-patent document 2] W. Wang, T. Ottosson, M. Sternad, A. Ahlen, A. Svensson, “Impact of multiuser diversity and channel variability on adaptive OFDM,” IEEE VTC2003-Fall, pp. 547-551, October 2003.